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UCSC OS 130 - Factors Affecting Photosynthesis

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Factors AffectingPhotosynthesis Temperature Eppley (1972) Light Sverdrup’s Critical Depth Model Nutrients Definition and Units Broecker’s Classification Scheme Limitations Uptake KineticsTemperature•The oceans vary much less than the landdoes, both seasonally and daily•Increased temperature decreases viscosity,so you sink•Organisms grow faster, die younger astemperature increases•In general, warm waterspecies are smaller andhave more extensionsTemperature &PhytoplanktonTemperature (deg C)Growth Rate03015• Eppley (1972)plotted speciesgrowth vs. temp.• Empiricallydetermined thatall phytoplanktonfit under a curveQ10Temperature &PhytoplanktonPBopt & TemperaturePBopt & TemperatureCompensation DepthZcPositive Net ProductionPositive Net RespirationSverdrup’s CriticalDepth Model:“…there must be a critical depth such that blooming canoccur only if the mixed layer is less than the critical value.”Assumptions:• Constant mixing, uniform phytoplankton• NO Grazers!• Nutrients are not limiting• The compensation depth is known• Production is directly controlled by lightand is linearCritical DepthGiven the previous assumptions,the Critical Depth (Zcr) can beapproximated by:Zcr / (1-e-k•Zcr) = Eo / (Ec x k)Zcr = Eo / (Ec x k)Nutrient DistributionsNutrient Availability Phytoplankton are most abundant wherethere are nutrients Nutrients are highest near coastal regionsand in upwelling zones Nutrients and waste products must passthrough the cell membraneDo Nutrients ReallyDiffuse? However, most phytoplankton cannot rely onpassive diffusion! Diffusion Mechanisms: Passive Diffusion (based solely on thegradient of concentrations) Facilitated Diffusion: “channels” allow ions tomove through the cell wall Active Uptake: There aretransporters on the cell wallUptake KineticsPassive Diffusion- Relies on a simple gradient- Not very efficientUptake RateFacilitated Diffusion- Provides “channels”Active Transport• Follows Michaelis-Menten Kinetics• Controlled by # of transportersAnd internal enzyme kinetics ConcentrationWho takes up nutrients thefastest?Stoichiometry depends on N source andchemical composition of phytoplanktonUnderstand and remember the definition andsignificance of the photosynthetic quotient, PQ1.0 NO3-+ 5.7CO2+ 5.4H2O  →  (C5.7H9.8O2.3N) + 8.5 O2+1.0 OH-P.Q. = 1.49 (O2evolved / CO2consumed)1.0 NH4++ 5.7CO2+ 3.4H2O  →  (C5.7H9.8O2.3N) + 6.25 O2+1.0 H+P.Q. = 1.10Generalized reactions for growth on nitrate and ammoniumIt is convenient (and often necessary) to consider the growthand decomposition of an “average” phytoplankter. Redfield(Redfield, Ketchum and Richards 1963) showed strong andprofound relationships between dissolved elements that wereconsistent with the growth and decomposition of phytoplankton:Growth on CO2 and theMacronutrients N and PNitrate and phosphate to proteins, phospholipids, nucleotides, etc.…the implicit PQ is 1.30€ 106 CO2+ 122 H2O +16 HNO3+ H3PO4 →  (CH2O)106+(NH3)16+H3PO4+138 O2C:N:P ~ 106:16:1 - Termed the Redfield RatiosMicronutrients (Trace Elements)e.g.,Cu, Zn, Ni, Co, Fe, Mo, Mn, B, Na, ClGenerally, these are required to act as cofactors in enzymes(Ferredoxin [Fe], Flavodoxin [Mn], Carbonic Anhydrase [Zn])Iron is well recognized as being in short supply over large partsof the ocean. It is particularly important in Nitrogen Fixation.Copper, Zinc and Nickel have also been implicated ininfluencing the growth of open-ocean phytoplankton. Traceelement interactions are complex, and incompletely understood.ChloroplastNADPH NADPGln + 2-OXG GluGlu + NH4+GluNIRFDX(red)FDX(ox)ADPATPGSGOGAT+ATPADP + PiNO3-NO3-[plasma membrane][bulk fluid][cytosol]aminoacidsNAD(P)HNAD(P)NRNO2-α ketoacidsMitochondrionTCACycle+Adapted from Falkowski and Raven (1997) Aquatic PhotosynthesisN-Metabolism is a Primary Sink For Photo-ReductantMichaelis-Menten kinetics:V = Vmax⋅SKs+ SV = uptake rate (e.g., N taken up per unit particulate N per unit time); d-1Vmax = maximum uptake rateKs = Substrate concentration at which V = Vmax/2Consistent with underlying mechanism:S + Ek1k–1k2E SE + PS = substrate; E = enzyme; P = product; k = rate constantNutrient-uptakekinetics andecological/evolutionaryselectionPhytoplankton isolated from oligotrophic environments havelower Ks values than phytoplankton from eutrophicenvironments (consistent with prediction based on ecologicaltheory)0.00.51.01.52.02.50 2 4 6 8 10 12Nutrient UptakeSpecific Rate of Uptake (d-1)Nutrient Concentration (µM)IIIVmax = 2.25 d-1Ks = 2.0 µMVmax = 1.5 d-1Ks = 0.5 µMNutrient kinetics for growth (rather than foruptake) are more difficult to determine:experiments involve growth in chemostatcultureKs < 0.1 µg-at L-1Droop Kineticsµ = µmax(1 - kq / Q)µ = growth rate kq = minimum cell quotaQ = current cell quotaQmax = max cell quota• If an organism has a high degree of “quota flexibility”, itcan vary the ratio kq/Qmax by quite a bit--this allows forluxury uptake• Redfield Ratios are ONLY approximated when µ/µmaxis close to 1• Therefore, cell composition can provide an indication ofcell growth status, or limitationConsequently, chemicalcomposition responds togrowth conditions0.000.020.040.060.080.10N:C molar ratio0.120.0 0.2 0.4 0.6 0.8 1.0µ (d-1)A189 µmol m-2 s-163 µmol m-2 s-1N-Limited <——> N-sufficientThe chemicalcomposition ofphytoplankton isvery responsive togrowth conditions.Here, nitrogencontent is lowerwhen growth rate islimited by the supplyof N (carbohydratesare accumulated).Photoacclimation affects chemical compositionELSHigh LightLow Lightafter Geider et al. 1996PLSEPSizes of arrows are proportional to flux:Sizes of boxes ∝ pool size × growth rateP = PhotosynthateE = EnzymesS = StorageL = Light HarvestingUnbalanced growth High —> LowLow —> Highsee Geider et al. 1996 LSEPPigment synthesis inhibitedSynthesis of enzymes cannot accelerate quicklyPhotosynthate goes to storageELSPPigment synthesis continuesSynthesis of enzymes slows because supply is reducedStored carbon is mobilized into free sugarsBiological and Solubility Pumps• Geochemists' viewpoint : nitrogen can be "topped up"from the atmosphere by the fixation of N2 gas to NO3;phosphorus has no comparable sources or biologicalpathways, therefore phosphorus limits global production• Biologists' viewpoint : observational and experimentalwork finds natural assemblages of phytoplankton are


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UCSC OS 130 - Factors Affecting Photosynthesis

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